TEXAS 49-year old married Woman and her brother in law - consensual?

Looking at affinity, incest and consanguinity in the State of Texas (Tarrant County), I am confused. A forty-nine year old woman (object) married to a seventy-two year old man is having consensual sex with her sister's husband (aka "brother-in-law"), a 40-something year old . These are NOT "open" marriages and the affair is secretive (it's NOT consensual between her and her husband NOR him and his wife), though I am privy to it. I am not one of the parties involved, except I care for the 49-year old woman and it is painful knowing this. The 49-year old woman has a past which includes addiction to men (or sex), lives emotionally unattached to, but physically present with her husband (same household). Her brother in law does NOT live with his wife and also has, for lack of a better term, a "Marriage Of Convenience". I believe the brother in law is using her for a sole purpose and has intertwined himself into her life by means of friendship of their daughters, but I'm having trouble getting her to recognize this and my trying to fight an active addiction in the 49-year old woman alone is fruitless, no? Clearly it is morally objectionable and all parties involved are risking alienation by everyone but is this LEGAL behavior ?